The Age of Water and Ventilation Timescales in a Global Ocean Model

Abstract

The age of water in the World Ocean is studied using a passive age tracer introduced into a global ocean model. Additional information is derived from a transient ?dye? tracer that tracks the time-dependent spreading of surface waters into the model ocean interior. Of particular interest is the nature of ocean ventilation over the 10?100-yr timescale, as well as the simulated age of deep and bottom water masses. In the upper model levels young water is found to correspond with regions of convergence (and downwelling) in the surface Ekman layer. Upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer. In the deep model levels, water age varies greatly between oceans, with young water found in convectively active regions (in the North Atlantic and in the Ross and Weddell Seas), and old water found in the deep North Pacific. The oldest water mass mixture (located at 2228-m depth in the western Pacific Ocean) is dated at 1494 years, made up of a combination of sources of water whose age varies between 500 and 5000 years. In the bottom layers of the model, Antarctic Bottom Water ventilates the extreme Southern Ocean over a 50?100-yr timescale, whereas the age approaches 1000 years in the northern limit of the Pacific basin. An analysis of age on the σt = 27.4 kg m?3 isopycnal surface shows North Atlantic Deep Water (NADW) leaving the Atlantic Ocean with an average age of 300 years, although part of this water mass mixture is as young as 60 years. The young signal of NADW penetrates the Indian Ocean and the South Pacific via the circumpolar current over a timescale as short as 15 years, although water penetrating the far deep North Pacific is not detected in significant quantities (using a 10% concentration criterion) until about 500 years after the NADW formation time. A volumetric census of age in the World Ocean model shows relative maxima at 2°?3°C, 1200 years (corresponding to water in the deep North Pacific), and at 3°C, 300?500 years (corresponding to water in the deep Atlantic Ocean). The parameterization of mixing in the ocean model partly determines age, with an isopycnal mixing scheme reducing the deep and bottom water ventilation timescale by about 30%. By monitoring the gradual penetration of surface dye into the most remote ocean grid boxes, the time taken to ventilate the entire World Ocean model can be estimated to be around 5000 years.